The present invention relates to control and actuator circuits for furnaces or similar gas burning equipment. The invention is more specifically concerned with a driver circuit for a gas valve will actuate a dual-solenoid gas valve in a fashion that will fail-safe when one component of the gas valve fails or sticks. The invention is also concerned with a circuit that actuates two or more solenoids from a single controller output.
In a modern gas furnace, one or more gas burners inject a gas flame through a heat exchanger, and the combustion gases are drawn through the heat exchanger by means of an inducer blower, which exhausts the combustion gases to a vent or flue. A pressure sensor associated with the inducer actuates a pressure switch to indicate a pressure differential between the exhaust and intake of the inducer. The pressure switch provides assurance that the inducer is functioning properly. There is also a flame sensor which is intended as a means for shutting the gas valve off if flame does not appear in the burner after some limited period of time.
An indoor air blower forces air from a comfort zone past the heat exchanger to draw heat from the combustion gases. The warmed air is then returned to the comfort zone. A temperature limit switch on the heat exchanger is normally closed, and opens if the heat exchanger exceeds a predetermined temperature. This limit switch serves as a check on proper air flow and functioning of the indoor air blower.
A thermostat located in the comfort zone closes when the room temperature drops below a predetermined setpoint, and thereby signals a call for heat. When a call for heat is detected, control and timing circuitry for the furnace actuates the inducer blower and then initiates an actuation sequence which energizes a gas valve relay so that current is supplied to the gas valve. This allows combustion gas to flow to the burners. At this time, igniters are actuated to light the burners, and the furnace begins to produce heat. An infrared detector, rectification or other mechanism is employed to ensure that there is flame after the gas valve is actuated. If no flame is present, another series switch interrupts the thermostat power and turns off the gas valve. Also, a rollback switch detects if flame is not entering the heat exchanger but is instead proceeding in the combustion air intake direction.
After the burners have been ignited for a predetermined time, the room air blower is powered up, and this creates a flow of warm air to the interior comfort zone.
Conventionally, 24 volt thermostat power, either AC or DC, is supplied through the series arrangement of the limit switch, thermostat, pressure switch, gas valve relay, and gas valve.
As aforementioned, the limit switch, thermostat, and pressure switch are all disposed in series with the gas valve relay, so that no current can flow through the gas valve relay to actuate the gas valve, until the limit switch and pressure switch are both closed. This serves as a check that the room air blower and the inducer blower are functioning properly.
A safety problem can arise if any of the limit switch, pressure switch, or gas valve relay are for some reason locked into a closed condition. In those cases, the gas valve will continue to feed gas to the burners if the heat exchanger experiences overtemperature, or if the inducer fails to produce sufficient draft.
This problem, and a solution to it, are described in Kadah U.S. Pat. No. 5,917,691 and in Kadah et al. U.S. Pat. No. 5,889,645.
Generally, whenever there is a call for heat, the controller should be able to check the conditions of the pressure switch and the gas valve relay before supplying current to the coil for the gas valve relay. This permits the control circuit to check for switch malfunction and indicate a service condition, if service or repair is required.
The gas valve relay has a pair of actuator coils, and these are generally connected in parallel. A gas valve driver circuit is controlled by an output of the microprocessor and supplies drive current to both coils. Each solenoid has an associated gas valve mechanism, and these are connected in series on the gas line so that both valve mechanisms have to open for gas to reach the burner. This is intended as a safety measure, and to ensure that one of the gas valves will close if, for some reason, the other sticks in the open position. Also, the gas valve relay driver circuit is typically designed so that if any of the driver components fail, the gas valve will not turn on. These safety features are necessary to keep the room space around the furnace from flooding with unignited gas, as that could present a danger of either suffocation or explosion. Normally, the two solenoids are tied together, so that the driver feeds both of them at the same time. However, an additional safety feature can be obtained by driving one of the two solenoids directly, and the other indirectly. In that way, if the first solenoid or its driver fuses or is locked up, the second solenoid will fail to actuate, i.e., will not turn on its associated gas valve mechanism. Unfortunately, no one has previously configured a gas valve driver to do this.